Shaded pole motors have been used for starting of small single-phase, alternating current motors, generally of the induction start, and whether induction-run or synchronous-run. The usual shaded pole motor has concentrated windings rather than distributed windings, so that there is a definite pole face which may be provided with a shaded portion. A slot leads in from the air gap between the stator and rotor to define the boundary between the shaded and unshaded sections, and a short circuiting ring surrounds the shaded section, having one coil side in the slot and one coil side on the exposed side of the pole piece, where it is present in the winding space for the stator coil. This construction is shown in U.S. Pat. Nos. 3,697,842; 3,959,678; and 4,234,810.
For safety, the exposed side of the short circuiting coil must be insulated in order to avoid a potential short on the stator coil, with its usual enamel insulation on the wire of the stator coil. This stator coil-to-ground insulation can be a preformed bobbin, as in U.S. Pat. No. 1,935,606, or it can be an insulation which is applied electrostatically or by heat and insulative powder to coat the exposed surface of the stator core, as in U.S. Pat. No. 4,234,810. Since the shading coil exposed side normally protrudes into the winding space, it is often difficult to make certain that the insulative powder, whether applied by heat or by electrostatic means, properly covers all surfaces, especially the sharp corners. It has been suggested that the shading coil be recessed into the exposed side, as in U.S. Pat. No. 2,355,175, so that the insulative bobbin has a relatively smooth surface on which to be mounted. U.S. Pat. Nos. 3,697,842 and 3,959,678 show the difficulty of properly insulating all exposed surfaces of the short circuiting shading coil and the difficulty of providing a winding behind that coil. U.S. Pat. No. 1,935,606 suggests that the shading coil be provided not in slots, but in apertures in a magnetically permeable ring for the purpose of being able to rotate the ring so that the motor is able to be started in either of the two rotational directions.